Effects of Reaction Kinetics and Conversion on Photopolymerization Stress Development

Efforts to reduce the stress associated with dental polymers used in composite restoratives have focused on new materials as well as use of modified photocuring protocols.  Regarding the latter, accurate characterization of polymerization rate and limiting conversion is essential for rational interpretation of any observed differences in stress.  Objective:  This study compares the effect of conversion, achieved under a variety of reaction conditions, on polymerization stress.

Methods: Stress specimens (n=3) prepared from a visible light initiated Bis-GMA/TEGDMA unfilled resin were irradiated at 100 mW/cm² for either 5, 10, 20, 40, 120 or 240 s or alternatively at 35 mW/cm² or 240 mW/cm² for 20 and 40 s, respectively.  Dynamic stress was measured using a cantilever beam tensometer while simultaneous real-time conversion was monitored using near-infrared spectroscopy.  The coupled conversion and stress data were collected over 15 minutes for each trial.

Results: This analytical approach allows stress evolution during polymerization to be plotted as a function of conversion so that effects of reaction rate and final conversion can be differentiated.  There is an obvious nonlinear development of stress with conversion in which stress rises dramatically beyond approximately 50% conversion.  Final conversion values ranged from 46.0 ± 2.0% (5 s at 100 mW/cm²) to 67.9 ± 0.1% (40 s at 240 mW/cm²), which coincided with the lowest and highest final stress levels observed (0.11 ± 0.06 and 2.77 ± 0.07 MPa, respectively).  Overall, the final stress increases in a predictable exponential manner (r²= 0.994) with final conversion and no deviation in the conversion dependence was evident regarding the polymerization rate.

Conclusions:  The technique of simultaneous stress and conversion evaluation provides critical fundamental information to assist in understanding polymerization stress development.  Under the conditions used here, final conversion and not reaction rate determines polymer stress.

Support: NIH/NIDCR 2R01DE14227 and a gift from Septodont.